1. Field of the Invention
The present invention relates to an imaging apparatus for capturing a fluoroscopic image of a subject.
2. Description of the Related Art
Flat-panel detectors that output digital data are now widely used instead of conventional screen/film systems in recent X-ray imaging systems. A flat-panel detector is so adapted that electric charge (referred to as “signal charge” below) proportional to X-ray dosage accumulates in each of a number of two-dimensionally arrayed detecting elements, and the accumulated signal charges are read out to a signal output line sequentially via TFTs (thin-film transistors) having a switching function. Signal charge that has been output to the signal output line is subjected to a charge-to-voltage conversion and is displayed upon being converted to a digital signal by an A/D converter.
An X-ray imaging apparatus can be classified broadly into three types, namely an upright type in which the subject is imaged in an upright attitude, an inclined type in which the subject is imaged while lying upon a bed, and a portable (cassette) type that can be moved about and can image a subject in any attitude.
In order to provide a guide for making an incision or to recognize the irradiation field when surgery is performed, it has been proposed to use an optical camera to capture the optical image of the subject surface in the imaging plane of the X-ray imaging apparatus, and to display the image obtained. Japanese Patent Laid-Open No. 54-158984 discloses a technique in which an optical image of the surface of a subject is captured by an optical camera via a half-mirror placed between an X-ray source and the subject so as to obtain an optical image having the same direction as the imaging direction of the X-ray imaging apparatus, with the captured image being displayed on a monitor.
The arrangement disclosed in Japanese Patent Laid-Open No. 54-158984 is premised on the fact that there will always be a stable relative positional relationship among the X-ray source, an X-ray detector that detects X-rays and an optical imaging unit for capturing the optical image of the subject surface. However, there is the possibility that this relative positional relationship will be disturbed, particularly in the case of a portable X-ray imaging apparatus.